Originally, then, it was comparatively homogeneous in consistence; and, becauseof the circulation which takes place in heated liquids, must have been comparativelyhomogeneous in temperature. It must, too, have been surrounded by an atmosphereconsisting partly of the elements of air and water, and partly of those variousother elements which assume gaseous forms at high temperatures. Cooling byradiation must, after an immense time, have resulted in differentiating theportion most able to part with its heat; namely, the surface. A further cooling,leading to deposition of all solidifiable elements contained in the atmosphere,and then to precipitation of the water, leaving behind the air, must thushave caused a second marked differentiation; and as the condensation commencedon the coolest parts of the surface-namely, about the poles there must sohave resulted the first geographical distinctions.
To these illustrations of growing heterogeneity, inferred from known laws,Geology adds an extensive series that have been inductively established.
The Earth' s structure has been age after age further complicated by additionsto the strata which form its crust; and it has been age after age made morevarious by the increasing composition of these strata; the more recent ofwhich, formed from the detritus of the more ancient, are many of them renderedhighly complex by the mixtures of materials they contain. This heterogeneityhas been vastly augmented by the actions of the Earth's nucleus on its envelope;whence have resulted not only many kinds of igneous rocks, but the tiltingup of sedimentary strata at all angles, the formation of faults and metallicveins, the production of endless dislocations and irregularities. Again,geologists teach us that the Earth's surface has been growing more variedin elevation -- that the most ancient mountain-systems are the smallest,and the Andes and Himalayas the most modern; while, in all probability, therehave been corresponding changes in the bed of the ocean. As a consequenceof this ceaseless multiplication of differences, we now find that no considerableportion of the Earth's exposed surface, is like any other portion, eitherin contour, in geologic structure, or in chemical composition.
There has been simultaneously going on a gradual differentiation of climates.
As fast as the Earth cooled and its crust solidified, inequalities of temperaturearose between those parts of its surface most exposed to the Sun and thoseless exposed; and thus in time there came to be the marked contrasts betweenregions of perpetual ice and snow regions where winter and summer alternatelyreign for periods varying according to the latitude, and regions where summerfollows summer with scarcely an appreciable variation. Meanwhile, elevationsand subsidences, recurring here and there over the Earth's crust, and producingirregular distributions of land and sea, have entailed various modificationsof climate beyond those dependent on latitude; while a yet further seriesof such modifications has been caused by increased differences of heightin the surface, which in sundry places have brought arctic, temperate, andtropical climates to within a few miles of one another. The general resultsare, that every extensive region has its own meteorological conditions, andthat every locality in each region differs more or less from others in thoseconditions: as also in its structure, its contour, its soil.
Thus between our existing Earth, the phenomena of whose varied crust neithergeographers, geologists, mineralogists, nor meteorologists have yet enumerated,and the molten globe out of which it was evolved, the contrast in heterogeneityis striking. §119. The clearest, most numerous, and most varied illustrationsof the advance in multiformity that accompanies the advance in integration,are furnished by living bodies. Distinguished as these are by the great quantityof their contained molecular motion, they exhibit in an extreme degree thesecondary re-distributions which contained motion facilitates. The historyof every plant and every animal, while it is a history of increasing bulk,is also a history of simultaneously-increasing differences among the parts.
This transformation has several aspects.
The chemical composition which is almost uniform throughout the substanceof a germ, vegetal or animal, gradually ceases to be uniform. The severalcompounds, nitrogenous and non-nitrogenous, which were homogeneously mixed,segregate by degrees, become diversely proportioned in diverse places, andproduce new compounds by transformation or modification. In plants the albuminousand amylaceous matters which form the substance of the embryo, give originhere to a preponderance of chlorophyll and there to a preponderance of cellulose.
